1. Field of the Invention
The present invention relates to a semiconductor photodetector element and a semiconductor device that includes the semiconductor photodetector element.
2. Description of Related Art
In recent years, various portable devices including cellular phone terminals have been developed as a product. These portable devices usually operate on batteries and are therefore requested to be low in power consumption. The power consumption of the portable devices has increased due to the sophistication of their functions, and the resultant shortening of battery life is a serious problem.
Those portable devices generally include an image display screen such as a liquid crystal display screen, and most of the power consumed by the portable devices is spent on displaying data on the screen. Equipping a portable device with a function of stopping displaying when there is no need to display on the screen is therefore an effective measure to reduce the power consumption. In a case of a cellular phone terminal, for example, automatically stopping the screen display operation while on call when there is no need to display on the screen reduces the power consumption and accordingly prolongs the battery life of the cellular phone terminal.
Usually, a cellular phone terminal is put close to the head of its user when the user is talking on the cellular phone terminal. Some conventional cellular phone terminals therefore employ a proximity sensor which is mounted in the vicinity of the phone's speaker and detects the distance between the phone terminal and the head of the user in order to determine whether the user is talking on the phone terminal or not. The proximity sensor includes a light emitting element, which uses a light emitting diode (LED) that emits near-infrared light, and a light receiving element, which uses a photodiode. Near-infrared light emitted from the light emitting element is reflected by the head of the user, and the light receiving element receives the reflected light, thereby detecting whether or not the head of a person is close to the speaker of the phone terminal. Detecting the head of a person near the speaker of the phone terminal, the proximity sensor controls the phone terminal such that displaying on the screen is automatically turned off.
In recent years, people have become more aware of global warming and other environmental concerns, and image display devices such as television sets, too, are demanded to be reduced in power consumption. The demand for lower power consumption is more intense for portable devices with a television function or a similar function, which are lately gaining popularity, because the portable devices operate on batteries. In portable devices with a television function or a similar function, using a proximity sensor as the one described above (ranging sensor) is being considered as a way to automatically turn off displaying on the screen when no one is around.
Another effective measure used to reduce power consumption is adjusting the brightness of the display screen automatically to a level suitable to the brightness of the surroundings. Specifically, an illuminance sensor which detects the brightness of the surroundings is mounted to automatically adjust luminance of the display screen based on illuminance data provided by the illuminance sensor and thus optimize the luminance of the display screen. In this way, a reduction in power consumption and improvement in display screen visibility are accomplished simultaneously.
The illuminance sensor commonly employed for this use is a photodiode formed on a semiconductor substrate. The illuminance sensor that uses a photodiode has sensitivity to invisible light such as infrared light in addition to visible light whereas the human eye is sensitive to light in the visible light range. Therefore, when ambient light contains infrared light, the influence of the infrared light creates a difference between the brightness perceived by the human eye and the brightness detected by the illuminance sensor. A conventional solution is therefore to remove the unnecessary infrared light by placing an interference filter film that blocks infrared light above the photodiode.
However, the interference filter film is manufactured by a laborious manufacture process in which evaporation on a glass substrate is repeated, and its multi-layer structure containing layers of a plurality of materials makes the manufacturing cost very high. Using the interference filter film thus poses a problem of increased cost in the manufacture of an illuminance sensor. Another problem is that the additional space required to dispose the interference filter film makes size reduction difficult.
There has been conventionally proposed an illuminance sensor that is lowered in sensitivity to infrared light without needing the interference filter film. This type of illuminance sensor is described in, for example, Japanese Patent Application Laid-open No. 2004-119713.
The illuminance sensor described in Japanese Patent Application Laid-open No. 2004-119713 lowers its sensitivity to infrared light by forming two photodiodes of different depths in a semiconductor layer such that the photodiodes overlap each other in the longitudinal direction, and computing detection signals of the respective photodiodes. In this illuminance sensor, a first photodiode is formed on a surface side of the semiconductor layer and a second photodiode is formed below the first photodiode. A detection signal of the second photodiode is multiplied by a given number and then subtracted from a detection signal of the first photodiode to lower the sensitivity to infrared light.
Inconveniently, the conventional illuminance sensor described in Japanese Patent Application Laid-open No. 2004-119713 has difficulties in reducing the difference in sensitivity to infrared light between the two photodiodes due to the differences in structure, conductivity type, impurity profile, and the like between the two photodiodes. The difference in sensitivity to infrared light between the two photodiodes makes it difficult to completely eliminate the influence of infrared light through computing. Thus, it is difficult to sufficiently lower the illuminance sensor's sensitivity to infrared light. Consequently, improving precision is difficult for the illuminance sensor described in Japanese Patent Application Laid-open No. 2004-119713.
Some cellular phone terminals and other portable devices require the functions of the proximity sensor and the illuminance sensor both to reduce power consumption. Mounting a sensor that has the proximity sensing function and a sensor that has the illuminance detecting function separately creates problems in terms of cost and packaging size. For that reason, the successful development of a semiconductor photodetector element with a proximity sensor and an illuminance sensor formed on the same chip has been sought after.
However, when a proximity sensor and an illuminance sensor are formed on the same chip, infrared light incident on the proximity sensor enters the illuminance sensor as well. This is a problem in the case where the illuminance sensor side of the semiconductor photodetector element takes the structure of the illuminance sensor described in Japanese Patent Application Laid-open No. 2004-119713, because the infrared light causes the illuminance sensor to malfunction.